As shown in FIG. 2 or 3, these coil units of the coreless armatures have been formed by winding a single coil element wire 1' sequentially in the directions indicated by arrows so as to be diamond-shaped or tapered and shaping it into the cylindrical shape by causing a coating of the coil element wire adhered integral without requiring an iron core. There have been such defects that, at it is necessary to wind up the coil unit with a single winding to the last, it must be made one by one and, therefore, the productivity is low. As the coil element wire is wound up as inclined, the overlapped parts increase with advances of the winding and the diameter will partly increase to cause the diameter of the coil unit to be partly varied. Thus, the coil wire length per turn is non-uniform and has a poor dynamic balance. When the coil element wire is flat, the wire cannot be freely bent as twisted so that a coil unit of coreless armatures of the flat coil element wire in section and small coil thickness cannot be obtained.
Causes of the poor dynamic balance and appearance shall be explained more in detail with reference to FIG. 4.
FG. 4 is a perspective view of an essential part of an example of the conventional coil unit of coreless armatures.
According to the winding formation shown in FIG. 2, first, as has been disclosed, the winding of the coil element wire is made in the order of a , b , c , and d in close contact as shown in FIG. 4 and is terminated in the order of 4 , 3 , 2 , and 1 after making one rotation along the circumference as overlapped on the wire previously wound. The coil element wire parts a , b , c , and d are held by turning parts 4 ', 3 ', 2 ', and 1 ' of 4 , 3 , 2 , and 1 and both groups are adhered to be integral.
The parts 4 and 4 ' of the coil element wire are held by the later wound parts 3 , 2 , and 1 ; the parts 3 and 3 ' are held by the parts 2 and 1 ; the parts 2 and 2 ' are held by the part 1 . The coil element wire parts 1 and 1 40 will no longer have any coil element wire to hold them from above when the turning part of the wire 1 and 1 ' is the last one, and this part is left in a state likely to float up (or to be easily separated). As a result of the foregoings, the coil unit obtained is poor in the dimensional precision of the inner and outer diameters and also poor in appearance.
Further, there have been such defects that, since the wire parts 1 ', 2 ', 3 ', and 4 ' are interposed between the wire parts a , b , c and d exposed below the above-overlapping wire parts 1 , 2 , 3 and 4 and a compression in the surfatial direction is required for making the thickness of the coil unit constant, they are caused to be abruptly bent in the overlapping parts as shown in FIG. 4 to become poor in the unbalanced and, at the same time, the coil element wire is subjected to a severe work or stressing.
The reason why the designing of a miniature direct current motor has been made inflexible shall be explained next with reference to FIGS. 5 to 7.
FIG. 5 is a plan view schematically showing a conventional coil unit of coreless armatures, and FIG. 6 is a view showing the conventional coil element wire.
Once the wire diameter of the coil element wire 1' is determined in this conventional cylindrical coil, the number of turns has to be limited in relation to the diameter of the coil unit 16' thus, such characteristics as the torque, number of revolutions and the like of the motor will not be able to be improved, whereby the designing of the motor is rendered inflexible. In order to improve the characteristics of the motor, the wire diameter of the coil element wire 1' has had to be varied. In the case where, for example, as in FIGS. 5 to 7, the wire diameter of the wire 1' is "d" and this wire 1' is wound by a winding method shown in FIG. 7(a) to form a coil unti 16' of a diameter D with a winding angle .theta. of the wire 1' with respect to an end edge of the coil unit 16', the number of turns N is represented by N=.pi.(p+d) sin .theta./d, the magnetic flux of the coil unit 16' is determined by the number of turns N. Therefore, the torque and number of revolutions of the motor are limited by the wire diameter d of the coil element wire 1'.